"Attacking the boats makes Israel a terrorist state".

Thousands of people came out across Scandinavia Monday to demonstrate against Israel's storming of a Gaza-bound aid flotilla that left at least nine pro-Palestinian activists dead.

"Attacking the boats makes Israel a terrorist state. They have attacked civilians," Yasser al-Sayed, in his 30s, angrily told AFP at a demonstration in Stockholm.

He was among up to 6,000 protesters crowding the central Sergelstorg, according to a police estimate, waving Palestinian flags and shouting "Boycott Israel!" and "Israel to The Hague!", and waving banners saying "There will be more ships to Gaza.".

"What happened today was terrible. I was very touched. That's why I had to come here," Taimi Tatti, 68, told AFP.

"When I heard there was this demonstration I sent emails to everyone I know, but I'm astonished how many people are here," she added as she prepared to march with the rest of the crowd through the centre of Stockholm to the Israeli embassy.

Similar demonstrations took place in a number of other Swedish towns Monday evening, with the country's second largest city Gothenburg gathering several thousand protesters, while around 600 people demonstrated in the third largest city Malmoe, in the far south.

In Denmark, hundreds of people showed up outside the Israeli embassy in the outskirts of the capital, waving banners demanding: "End the massacre of civilians!" and "Liberate Palestine!".

"I don't understand how Lego bricks can constitute a risk for Israeli security, or incubators for infants who are suffering from the blockade," said Kamal Qureshi, a doctor and spokesman for the leftist opposition Socialist Peaople's Party.

Denmark must demand an "immediate lifting of the absurd blockade" on the Gaza Strip, he said to thundering applause, blasting Monday's "unacceptable attack."

Several hundred demonstrators also gathered outside the Israeli embassy in Oslo, some burning an Israeli flag.

"We have lost many friends over the past 24 hours, and we have witnessed a terrible crime," Erling Folkvord of the far left Red party said in an address to the crowd.

"A government-authorised terrorist act using commando soldiers to attack civilian aid vessels and civilian personnel," said Folkvord, who had been scheduled to be onboard one of the ships in the flotilla.